1. Field of the Invention
The invention relates to an image compression technology. More particularly, the invention relates to a bit rate control technology for image compression.
2. Description of Related Art
Image compression technology effectively reduces memory occupied by an original image in order to save storage capacity but keep image contents. The image compression rate normally varies with complexity of image contents. However, for the application on digital camera to compress the static image, the occupied memory size is intended to be reduced but also the sizes for all the compressed images are also expected to have about the same size substantially not changing with the complexity of image content.
To maintain the image size, bit rate control of static image compression plays an important role, such that the image size of the static images is not significantly changed along with the image contents, and that a user can be aware of the remaining image storage capacity.
In U.S. Pat. No. 5,594,554, it is disclosed that a quantization table to be set in a second compression can be obtained based on a default algorithm model and the size of an image on which a first compression is performed. However, the default algorithm model achieves different levels of accuracy when this model is applied to different images. Moreover, the algorithm model adopted in this patent is rather complicated because logarithm calculation is implemented.
In U.S. Pat. No. 5,677,689, it is disclosed that a quantization table to be set in a second compression can be obtained based on a default algorithm model and an activity metric generated in a first compression. Nonetheless, the file compressed in a second compression can merely be smaller but not greater than the file compressed in the first compression.
In the related art, it is also disclosed that a factor ρ is defined as a ratio of the number R of zero coefficients to a total number T of coefficients after quantization. The factor ρ can be relevant to the size of compressed images.